The present invention relates to a method of producing a sheet of grain-oriented electromagnetic steel, particularly a sheet of grain-oriented electromagnetic steel with an improved watt loss property, as well as to the grain oriented electromagnetic steel sheet produced by such method.
There are two kinds of the grain-oriented electromagnetic steel sheets. However, only one kind is industrially produced for employment as the core material of transformers and various electric devices, and that kind is crystallographically designated as having a (110) [001] structure. This designation indicates that the (110) plane of the crystal grains of the steel sheet is parallel to the sheet surface, while the [001] direction of easy magnetization is parallel to the rolling direction of the steel sheet. In the actual steel sheets, the (110) plane of the crystal grains is deviated from the sheet surface, although at only a slight angle, and the [001] direction of the crystal grains is also deviated from the rolling direction at a slight angle. Since the excitation property and watt loss of the electromagnetic steel sheets are largely influenced by the degree of deviations mentioned above, a considerable amount of effort has been put into approximating the crystallographic orientation of all the crystal grains in the ideal (110) [001] orientation. As a result, it is currently possible to industrially produce electromagnetic steel sheets with a low watt loss of W17/50, which is equal to approximately 1.03 W/Kg with regard to a 0.30 mm thick sheet. The designation W17/50 indicates the watt loss under a condition of 1.7T of magnetic flux density and a frequency of 50 Hz.
Successive studies of electromagnetic steel sheet clarified that a prominent decrease of watt loss to a value lower than the value mentioned above cannot be achieved exclusively by means of approximating the crystal grains in the ideal orientation. Generally speaking, watt loss is dependent upon not only the excitation property, but also the crystal grain size of electromagnetic steel sheets. An excessive growth of crystal grains has been usually experienced in the prior efforts to improve the excitation property, and this has a tendency to counterbalance the amount of reduction in watt loss due to the improvement of excitation property. In short, it is not easy to achieve a prominent reduction in watt loss by conventional metallurgical means. Unless means different from metallurgical means for improving the watt loss is provided, the watt loss cannot be improved to a value lower than the conventional level.
It is known from U.S. Pat. No. 3,856,568 that one of the non metallurgical means for improving watt loss is to apply a tensile force to the steel sheets. As a means of applying tensile force, an insulating film is formed on the steel sheets. However, since the tensile force applied by means of the insulating film is limited, the watt loss value can be reduced to only about 1.03 W/Kg as a minimum, even by the aid of the tensile force effects.
Another non metallurgical means is known from U.S. Pat. No. 3,647,575. According to this patent, sharp scratches are formed on the surface of steel sheets by a knife, a blade of a razor, powder emery, a metal brush or the like. The watt loss reduction of a single sheet by the scratches can in fact be expected. However, since this process relies on a mechanical means, rising edges of unevenness are inevitably created on the sheet surface. Because of the intense unevenness as mentioned above not only is the space factor of the laminated sheets greatly decreased but also, the magnetostriction of the sheets is greatly increased. In addition to such disadvantages, there may arise such a serious disadvantage that a predetermined level of watt loss cannot be achieved with regard to the laminated sheets. In other words, the Epstein measurement value of the laminated sheets can be higher than a value measured by SST (measuring device for a single sheet). The reason for the watt loss reduction of the laminated sheets is understood to reside in the fact that the sheet thickness is locally reduced at the identations of the scratches in the steel sheets, and hence, a part of the magnetic flux emanates from each of the steel sheets via the indentations into adjacent, upper and lower sheets. As a result, the watt loss deteriorates due to the thus generated magnetization component, which is perpendicular to the steel sheets. The method of mechanically forming the scratches on the surface of the steel sheets is not advisable when the sheets form a core of laminated steel sheets, for the reasons explained above and, therefore, is difficult to adopt practically.
As still another non metallurgical means, a method for mechanically applying minute strain on the surface of steel sheets is used to improve the watt loss. As is well known, the watt loss is divided into a hysteresis loss and an eddy current loss, which is further divided into a classical eddy current loss and anomalous loss. The classical eddy current loss is caused by an eddy current induced due to a constantly changing magnetization in a magnetic material and results in a loss of the magnetization as a heat. The anomalous loss is caused by the movement of the magnetic walls and is proportional to the square of the moving speed of the magnetic wall. Since such moving speed is proportional to the moving distance of the magnetic walls when the frequency of the external current is constant, the speed and, thus, the anomalous loss are increased with the increase in the width of magnetic domains. However, with the increase in the width of magnetic domains and, thus, the decrease in the number of magnetic walls, the anomalous loss is not proportional to the square of the width of the magnetic domains, but is approximately proportional to the width of the magnetic walls. The anomalous loss accounts for approximately 50% of the watt loss at a commercial frequency of 50 or 60 Hz, and the proportion of anomalous loss is increased due to the recent development of decreasing eddy current and hysteresis losses of grain oriented electromagnetic sheets. Since narrow magnetic domains are important for the decrease of the anomalous loss, a tension force is applied to the sheets, from which the surface film is removed, so as to decrease the width of the magnetic domains.
The prior art includes U.S. Pat. No. 3,990,923, which proposes to insert between the conventional, decarburization and final annealing steps an additional step of locally working the steel sheet, so as to alternately arrange on the sheet surface the worked and non worked regions. The additional working step may be carried out by local plastic working or a local heat treatment by radiation utilizing infrared rays, light rays, electron beams or laser beams. The regions worked by plastic working or heat treatment serve to inhibit the secondary recrystallization of the steel sheet during the final high temperature annealing. In the worked regions the secondary recrystallization starts at a temperature lower than in the non worked regions and, thus, the worked regions function to inhibit the growth of secondary recrystallization grains produced in the non worked regions.